Recent history of game racket technology has as its focus a number of critical issues regarding the playability of stringed rackets. Since the advent of the oversized tennis racket, great emphasis has been placed on the size and location of the "sweet spot". Simply put, the "sweet spot" is the zone of optimum response on a stringed hitting surface. Direct contact with a game ball in this zone is likely to result in maximum power and control with reduced effort and reduced physical discomfort associated with off-center contact. Power and control are, at least in part, on the degree of rebound of a ball that strikes the stringed surface, i.e., the velocity of an incoming ball compared to the velocity with which the ball leaves the stringed surface, known as the "coefficient of restitution." The sweet spot is directly related to the rebound of a ball at different locations on the stringed surface, where rebound is at a minimum near the edges of the stringed surface, i.e., near the ends of the strings.
Efforts to expand the sweet spot with various modifications to the size and shape of the head and to the density and type of the string weave have been somewhat successful. However, the sweet spot of a conventional game racket still remains limited to a relatively small percentage of its stringed surface. A major drawback common to these previous efforts can be defined as a "fixed node" stringing system. "Fixed node" describes the condition of the string ends or nodes as secured to the racket frame about the periphery of the hitting surface. The fixed node stringing system is responsible for the limited size of the sweet spot and the lack of rebound near the edges of the racket. Consequently, any attempt to enlarge the sweet spot of a conventional racket is limited by the nature of the fixed node.
String tension can radically affect the playing characteristics of a game racket. Factors considered in determining a suitable tension can include string type and elasticity, racket frame stiffness, and playing conditions to name a few. More often than not, these factors vary from match to match. It is, therefore, desirable for a game racket stringing system to provide for tension adjustability without the need for a complete restringing. Conventional attempts to achieve this have utilized complicated hardware which often added undesirable weight to the racket. The adjustable tension racket of von Hackewitz U.S. Pat. No. 4,995,608 employs a cumbersome nut and bolt assembly that induces its adjustment from a centralized location such that the resulting tension on the stringed surface becomes uneven. This mechanism also requires that the head be discontinuous and susceptible to distortion and improper shock distribution.
A common goal of many game racket designs is the reduction or isolation of impact shock or vibration which increases with off-center contact. This unwanted shock or vibration is believed to be a cause of "tennis elbow" and other types of arm related injuries. Numerous attempts have been made to isolate and dampen the shock by various means of impact attenuation. These post-reaction oriented approaches, however, make no attempt to eliminate the transfer of impact shock to the frame, but respond only to the shock as transmitted from the stringed surface to the frame after it occurs. The anti-shock air cushions of Liu U.S. Pat. No. 5,083,776 are a good example of this trend. The cushions, located at the periphery of the racket head between the secured ends of the strings are intended to dampen the response of impact. This approach still results in high levels of vibration in the racket frame. Still other approaches have utilized shock absorption mechanisms in the handle grip. Nonetheless, all of these devices are designed to compensate for the inherent characteristics of the fixed node stringing system.
Another conventional approach to shock reduction is disclosed by Maynard U.S. Pat. No. 4,772,021 in which a stringed inner head frame of the "fixed node" type is suspended within the head of a racket by means of an inflatable rubber tube. The flexible inflatable tube is the sole source of lateral stability and does not provide sufficient resistance to its own tendency to roll. Thus, the extreme pressure necessary to stabilize the inner frame results in excessive clearances which render the racket inconvenient and impractical to use.
Elasticity of the string itself is also a serious consideration in the making of a game racket. It is disclosed as an object of Head U.S. Pat. No. 3,999,756, the oversized racket, for the string response to imitate as closely as possible, the performance of animal gut while making use of a far more durable yet less elastic synthetic string. To accomplish this, Head increased the length of the strings by enlarging the racket head size. This approach does not correct for the lack of rebound close to the fixed nodes at the periphery of the racket. Recent development of more durable and less elastic string, notably the wound steel type, increases the desirability of a racket that enhances the rebound of stringed surface and improves shock absorbency.
The ability of a string to maintain its tension over a period of time, is a major factor in its durability and playability. As a string begins to stretch and lose tension, it tends to rub across the adjacent strings more easily thereby increasing the chance that the string will be weakened and break. The option of decreasing the tension in the strings will minimize stretching and thereby increase the longevity and tension retention of the string. Thus, string longevity and tension retention would be a desirable game racket characteristic.
The preferred embodiment of this game racket with an adjustable air pressurized string suspension system offers a wide range of enhanced playing characteristics. Replacement of the fixed node system with a "suspended node" system results in a major increase in the size of the sweet spot, a significant increase in dwell time (actual time that a game ball has contact with strings) owing to the enhanced playing characteristics of the stringed surface, and a decrease in impact shock transmitted to the racket frame. Also improved are string playability and durability to name a few of the improvements. Add to these benefits a tension adjustment capability and it becomes clear that the present invention signifies a major breakthrough in game racket technology.
The numerous advantages achieved by the present invention can be attributed to the suspended node or air pressurized configuration of the stringing system. "Node" refers to the point at which the string is secured to the periphery of a racket frame head. Typically a stringed hitting surface is woven directly to the frame. Though the given tensile force in the string may be consistent, the rebound characteristic is not. The midpoint of the string span tends to offer greater rebound than its secured ends. "Suspended node" refers to a non-fixed or suspended point of string bearing which, in this particular embodiment, is movable in two opposite directions generally along a string axis. The suspended node allows a reduction in the rebound difference at a strings midpoint compared to near its termination at the periphery of the racket head.